1. Field of the Invention
The present invention relates to method and apparatus for accomplishing size reduction, classification, and separation of carbonaceous and noncarbonaceous mineral matter, and more particularly to method and apparatus for reducing the size of the mineral matter and separating the valuable constituent from the gangue.
2. Description of the Prior Art
Over the past years substantial research has been conducted on the subject of utilizing domestic coal to provide a reliable, low-cost source of clean fuel. As a result of the research it has been found that the major drawback to the use of coal as a fuel is the mineral matter impurities present in it. It is recognized that the development of a low-cost, highly beneficiated clean coal will provide an attractive alternative for the energy market.
Known beneficiation processes generally concentrate or improve the physical and chemical properties of the raw material by removing a large fraction of the impurities present in the mineral. Minerals and ores, e.g., oil shale, copper ore and iron ore have also been beneficiated in order to produce feedstock for recovering the shale oil or extracting the copper and iron. However, there is a need to enhance the efficiency of the size reduction and separation devices so as to produce the minerals and clean fuels at a cost and in a form that is economically attractive.
In conventional ball mills, the material to be ground is introduced into the mill at one end and the ground fine product is removed at the other end. The grinding media in the mill is alternatively stirred, vibrated, or tumbled to provide the energy necessary to grind/fracture the material to finer sizes.
A ball mill grinding apparatus as disclosed in U.S. Pat. No. 594,294 includes a ported mixer paddle whereby water is supplied to the mixing chamber. The movement of the metal balls through the injected slurry serves to reduce the size of the material particles. Other similar patents, namely Soviet Patents 3,149,789; 3,450,356; 3,226,044; and 3,486,705 disclose method and apparatus using liquid and grinding balls in combination with stirring paddles to process the injected solid matter.
In a conventional agglomeration beneficiation scheme, the material, such as coal, is comminuted to a predetermined average size such that the majority of the particles are liberated for separation. The coal in a water slurry is then introduced into a series of tanks and subjected to high and low shear agitation in the presence of the agglomerant. The agglomerated coal is separated from the refuse in the aqueous phase. The clean coal agglomerates may be subjected to a second separation step to separate the mineral matter which tend to get entrapped in the agglomerates.
A known separator is a froth flotation cell where the hydrophobic particles e.g. clean coal, are separated from the hydrophilic particles, e.g. pyrite and other ash forming minerals. However, if the separation is not efficient the refuse may contain a high organic content or the clean coal may contain a higher than required mineral matter (ash) content. Consequently, in order to correct the problem one or more of the following operations must be additionally performed: (1) reduce the average particle size of the feed, (2) comminute and reprocess the middling, or (3) change the density of separation. Variations in the feed coal quality, as a result of changes in the in-seam coal quality, may result in higher than normal losses in organic recoveries if product quality is maintained.
A conventional gravity separation circuit uses heavy liquids, organic or inorganic, for achieving the separation of the coal and associated mineral matter. In the circuit, coal is reduced to a predetermined size and separated in a cyclone, using heavy liquids of a known density. If the middlings contain a high organic content which would result in low coal recoveries, then they are reduced to a finer size and subjected to two or more heavy liquid separation stages to obtain a final product which meets grade and recovery specifications.
In a froth flotation cell, air is diffused into the slurry in the form of fine bubbles. Sufficient mixing is provided so as to enhance the probability of the hydrophobic particles striking and attaching themselves to an air bubble. A relatively quiescent zone is provided so that the froth is formed and removed. In general the two main objectives are: (1) to achieve a high recovery of the hydrophobic species into the froth product and (2) to retain the maximum amount of the hydrophilic species in the slurry. For the beneficiation of fine and ultrafine particulates a device is provided for the generation of extremely fine or microbubbles, resulting in greater selectivity in the process.
The figure below diagrammatically illustrates a beneficiation circuit, utilizing the froth flotation separation process, consisting of various unit operations for the production of low-ash coal. The cumulative effect of inefficiencies in controlling and operating each of the several unit operations results in designing a larger circuit with reduced recoveries and leads to higher operating and capital costs. ##STR1##
Many efforts have been made to improve the beneficiation of coal and other materials by means of improving the froth flotation process. For example, U.S. Pat. No. 4,326,855 teaches a beneficiation process whereby "clean slurry" is ground and cavitated by sonic energy removing any contaminants including iron pyrites and ash.
U.S. Pat. No. 3,746,265 teaches a gravity beneficiation process whereby the source comminuted potash ore is subjected to gravity separation and subsequently subjected to froth flotation. U.S. Pat. No. 4,593,859 also discloses a gravity separation beneficiation process.
U.S. Pat. No. 4,564,369 teaches an apparatus whereby coal is pulverized and its surface is treated by contact with chemical reactants for ultimate removal of hydrophillic impurities.
U.S. Pat. Nos. 4,597,858; 4,564,369; 4,597,857; and 4,593,859 disclose processes or methods for improvements of the beneficiation and/or separation of valuable material from its impurities.
Overall the known processes, such as sink and float separation, froth flotation, emerging selective agglomeration, and selective flocculation techniques require that the steps of size reduction of the material and the separation of its constituents be performed in separate chambers. Therefore, there is a need for a process whereby all the operations of size reduction, mineral classification and separation take place simultaneously in a single vessel in order to achieve higher separation efficiencies and reduced energy, chemical and space requirements. By making the beneficiation process more efficient and cost effective it will enhance the competitive position of clean and ultra-clean coal in the energy marketplace.